It is known that the bitumen-based membranes of the type described above have a multilayer structure comprising: a central supporting layer which is produced by means of a film made of flexible material trapped between two outer bituminous layers, an upper layer and a lower layer, opposite each other. Some types of bituminous membranes used to waterproof roofs have a larger surface covered by an outer granular layer which, in addition to protecting the membrane from atmospheric agents, is distributed on the surface to form a predefined decorative design, which generally represents the modular elements traditionally used for covering roofs, for example shingles, or flat or curved tiles.
The above-mentioned surface decoration of the membrane can be provided by different methods, some of which entail rotating an application drum provided with surface cavities, moving the bitumen-based membrane below the application drum, filling the cavities of the drum with covering grains, temporarily maintaining the covering grains in the cavities until the latter reach a certain angular position in which the grains are released from the cavities and are thus deposited by gravity on the surface to be decorated of the membrane below.
U.S. Pat. No. 5,812,369 describes a method of the type described above in which the grains are retained in the cavities by means of a flexible belt wound in a ring around a plurality of wheels and arranged at the back of the application drum so as to superficially envelop the semi-circumference of the drum arranged on the side where the grains are deposited, thus temporarily occluding the cavities during the rotation of the application drum.
The technical problem of this solution is that, during the feeding phase, some grains are deposited on the outside of the cavities and remain trapped between the belt and the surface of the application drum and are deposited in an uncontrolled manner on the membrane, causing irregularities/blurring on the lines/contours of the decoration which therefore spoil the decorative pattern. Furthermore, the grains trapped between the drum and the belt superficially abrade both the application drum and the belt itself. If said abrasion is continuous, in the long term it will damage both the drum and the belt which consequently have to be replaced, resulting in increased costs in terms of production, maintenance, machine standstills, etc.
WO 2015 125 089 A1 also describes a method of the type described above, in which the grains are temporarily retained in the cavities prior to depositing thereof by means of an occlusion roller made of elastically deformable material, instead of a flexible belt. The occlusion roller is positioned in contact with the application drum on the side on which the grains are deposited, thus partially deforming on the same side and therefore forming a sliding elastic surface portion which compresses the grains inside the cavities and cleans the surface of the drum during the rotation thereof.
The technical problem associated with this solution is represented by the fact that the occlusion roller, being made of deformable elastic material and being continually pressed against the drum, is subject to rapid wear by the latter and consequently has to be replaced often, with all consequent drawbacks.
Furthermore, depositing of the grains is often inaccurate. In fact, positioning of the roller axis on the horizontal centreline plane on which the drum axis lies and sizing of the sliding elastic surface portion which extends below the horizontal centreline plane are such that discharge of the grains from the cavity can commence when the cavity is positioned in the lower portion of the roller, well below the horizontal centreline plane thereof. In this way, the cavity is free to discharge when it is already substantially facing the membrane. Consequently, the initial discharge of the grains is carried out in an uncontrolled manner, i.e. not gradual, and can cause blurring on the lines of the decoration which impair the aesthetic effect thereof.
The Applicant has therefore carried out a detailed, study with the aim of identifying a simple and inexpensive solution for implementing a method to produce a bitumen-based membrane superficially covered by grain-based decorations which overcomes the technical problems highlighted above.